FIG. 12 shows a configuration of an image forming device 700 with a conventional image exposure device.
Referring to FIG. 12, arranged in image forming device 700 are a photosensitive drum 701, as an electrostatic latent image holder; a corona charger 702, which charges a surface of photosensitive drum 701 around drum 701 in the direction in which drum 701 rotates; an exposure device 700 having an optical shutter array (also referred to as an optical shutter head) 703 which exposes an image by one line on the charged surface of photosensitive drum 701 and an optical shutter driver 704 which drives optical shutter array 703, a developer 705 which develops an electrostatic latent image with a toner, a transfer/separation charger 706 which transfers a toner image obtained by the development onto a recording sheet, a cleaner 107 which removes residual toner on the photosensitive drum 701, and a residual charge elimination lamp 108 which illuminates the photosensitive drum 701 and removes residual charge of the photosensitive drum 701. A recording sheet 709 is transported by a transportation roller 710 between photosensitive drum 701 and transfer/separation charger 706.
Optical shutter array 703 is a solid scanner component having a plurality of optical shutter elements arranged in the longitudinal direction along the rotation axis of photosensitive drum 701.
FIGS. 13A and 13B show arrangements of optical shutter elements of optical shutter array 703 shown in FIG. 12.
Referring to FIGS. 13A and 13B, optical shutter elements of optical shutter array 703 are those formed of liquid crystal, PLZT (Pb Lanthanum-added Zirconate Titanate) or the like which are arranged in one line, as shown in FIG. 13A, or staggered in two lines, as shown in FIG. 13B, such that the number thereof corresponds to a width to be recorded on a recording sheet.
FIG. 14 is a cross sectional view of a configuration of a PLZT optical shutter head 900 as an example of optical shutter array 703 shown in FIG. 12.
Referring to FIG. 14, PLZT optical shutter head 900 includes a light emitting portion 30, a rod lens 31 which collects a linear pencil of light from light emitting portion 30, an optical shutter portion 32 which selectively transmits light from rod lens 31, and a rod lens array 33 which converges light transmitted from optical shutter portion 32.
Light emitting portion 30 includes a halogen lamp 34 as a point source of light, and a fiber optic light guide 35 which converts a pencil of light from the point source of light to a linear pencil of light.
Optical shutter portion 32 is constituted by a polarizer 36 which selectively transmits only the light in a certain polarization direction of incident light having a random plane of polarization and an analyzer 38 which transmits only the light in the polarization direction angled by an angle of 90.o slashed. relative to the polarization direction of the light transmitted from polarizer 36, with a PLZT optical shutter array 37 having electro-optic effect disposed therebetween, an optical shutter element of PLZT optical shutter array 37 having micropixels.
When a voltage is applied to an optical shutter element having micropixels and forming the PLZT optical shutter array 37 in the optical shutter portion 32, a plane of polarized light, transmitted from polarizer 36, is rotated and then passes through analyzer 38. On the other hand, when a voltage is not applied to an optical shutter element, a plane of polarized light, transmitted from polarizer 36, is unchanged and hence blocked by analyzer 38. Light is selectively transmitted depending on whether or not voltage is applied to each of optical shutter elements forming PLZT optical shutter array 37.
In an image forming device as described above, a driving voltage for an optical shutter element is preset. At the driving voltage, a period during which the optical shutter is opened is predetermined depending on the output pixel tone and thus it is operated.
However, as an accumulated drive period, i.e., an accumulation of periods during which an optical shutter is opened, is increased, durability of the optical shutter element is degraded and the quantity of light tends to be gradually decreased.
FIG. 6 shows how the amount of light emission of an optical shutter element changes relative to an accumulated optical shutter element drive period.
Referring to FIG. 6, in a conventional exposure device in which as the accumulated drive period is increased, the amount of light emission is decreased due to durability degradation of an optical shutter. When a predetermined period elapses, an accumulated drive period of an optical shutter element for a pixel A arranged in the longitudinal direction differs from that of an optical element for a pixel B arranged in the longitudinal direction, for example, depending on the image patterns which have been output and a drive period difference .DELTA.t is caused. That is, if there is a drive period difference of .DELTA.t between the optical shutter elements corresponding to pixels A and B, as shown in FIG. 6, there also is a difference in the amount of decreased light emission (the amount of degradation) and thus the difference .DELTA.E in the amount of light emission is caused.
Thus, since a plurality of optical shutter elements arranged in the longitudinal direction each have different accumulated drive periods, the reduced amount of light emission is different for each optical shutter element, thus causing unevenness in the quantity of light.
FIG. 15 is a state diagram illustrating a relationship between the period during which an optical shutter is opened (referred to as an optical shutter open period hereinafter) and the quantity of light outputted.
Referring to FIG. 15, in a conventional image forming device, which is assumed to be free from degradation and thus ideal, the optical shutter open period and the quantity of light are assumed to satisfy a linearly proportional relationship designated by the letter a and thus an optical shutter element open period corresponding to a tone is fixed independently of the drive period of each optical shutter element. In practice, however, the quantity of light is decreased due to degradation, as designated by the letter b, and thus it can be difficult to reproduce a pixel in a tone of interest.
If a voltage which drives an optical shutter element (referred to as a driving voltage hereinafter) is increased to compensate for the reduced quantity of light to achieve the state designated by the letter a, then the relation between the optical shutter open period and the quantity of light transmitted from the optical shutter will become nonlinear. If an optical shutter element is extremely degraded, deficiency in the quantity of light can be caused as the optical shutter open period reaches or exceeds T.sub.1, as designated by the letter c. If an optical shutter element is not so degraded, increase in the driving voltage can lead to saturation of the quantity of light, as designated by the letter d.
Thus, increasing the driving voltage to output a pixel in a multivalue tone cannot compensate for a change of the quantity of light due to durability degradation of an optical shutter element, since the optical shutter open period is preset for each pixel. Thus, a half tone or the like can not be reproduced successfully, and an image of high quality cannot be obtained.